The present invention is related to a transconductance-capacitance filter system equipped with a transconductance-capacitance filter circuit and an adjusting circuit thereof.
For instance, transconductance-capacitance filter systems (will be referred to as a xe2x80x9cgm-C filer systemxe2x80x9d hereinafter) are employed in portable electronic appliances such as portable telephone sets. FIG. 7 schematically represents an example of conventional gm-C filter systems. This gm-C filter system is equipped with a transconductance-capacitance filter circuit (will be referred to as a xe2x80x9cgm-C filter circuit hereinafter) 1, and an adjusting circuit 16 for adjusting a cut-off frequency of this gm-C filter circuit 1. The gm-C filter circuit 1 is arranged by employing a transconductance amplifier (will be referred to as a xe2x80x9cgm amplifierxe2x80x9d hereinafter) 1a and a capacitor 1b, and may constitute, for example, a low-pass filter. On the other hand, the adjusting circuit 1b is provided with an oscillator 3 having a gm amplifier 3a and a capacitor 3b, comparators 14 and 15 designed for waveform shaping operation, and a frequency comparator 13. In this case, the gm amplifier 3a of the oscillator 3 owns the same structure as that of the gm amplifier 1a of the gm-C filter circuit 1.
In the gm-C filter system arranged in the above-explained manner, an oscillation signal OSC is supplied from the oscillator 3 via the comparator 14 to the frequency comparator 13, and also a reference clock signal CK is supplied from an externally provided crystal oscillator (not shown) via the comparator 15 to this frequency comparator 13, so that the frequency of the oscillation signal OSC is compared with the frequency of the reference clock signal CK. In other words, in the frequency comparator 13, a bias current xe2x80x9ciBIASxe2x80x9d is produced based upon a frequency error of the oscillation signal OSC with respect to the reference clock signal CK. This bias current xe2x80x9ciBIASxe2x80x9d is supplied to the gm amplifier 3a employed in the oscillator 3 so as to adjust a value of a transconductance (will be referred to as a xe2x80x9cgm valuexe2x80x9d hereinafter) of the gm amplifier 3a. For example, in such a case that the oscillation frequency of the oscillator 3 is higher than the frequency of the reference clock signal CK corresponding to the set value, such a bias current xe2x80x9ciBIASxe2x80x9d capable of reducing the gm value of the gm amplifier 3a employed in the oscillator 3 is outputted from the frequency comparator 13, so that the oscillation frequency of the oscillator 3 is reduced. Conversely, in such a case that the oscillation frequency of the oscillator 3 is lower than the frequency of the reference clock signal CK corresponding to the set value, such a bias current xe2x80x9ciBIASxe2x80x9d capable of increasing the gm value of the gm amplifier 3a employed in the oscillator 3 is outputted from the frequency comparator 13, so that the oscillation frequency of the oscillator 3 is increased. In other words, the bias current xe2x80x9ciBIASxe2x80x9d is varied in such a manner that the oscillation frequency of the oscillator 3 is made coincident with the frequency of the reference clock signal CK, so that the gm value of the gm amplifier 3a employed in the oscillator 3 is adjusted.
On the other hand, the bias current xe2x80x9ciBIASxe2x80x9d supplied from the frequency comparator 13 is also supplied to the gm amplifier 1a provided in the gm-C filter circuit 1 so as to adjust the gm value of this gm amplifier. As a result, the cut-off frequency is adjusted. In this case, since the gm amplifier 3a of the oscillator 3 owns the same structure as that of the gm-C filter circuit 1, the oscillation frequency of the oscillator 3 may correspond to the cut-off frequency of the gm-C filter circuit 1 in an one-to-one correspondence relationship. As a consequence, in order to set the cut-off frequency of the gm-C filter circuit 1 to a desirable frequency value, the oscillation frequency of the oscillator 3 may be adjusted based upon such a frequency clock signal CK having a frequency corresponding to this desirable frequency value.
However, in the above-explained conventional gm-C filter system, there is such a serious problem. That is, the adjusting circuit 16 arranged by the oscillator 3, the comparators 14/15, and the frequency comparator 13 is continuously operated so as to adjust the cut-off frequency of the gm-C filter circuit 1. Since this adjusting circuit 16 is continuously operated, the power consumption of the entire gm-C filter system would be increased.
The present invention has been made to solve such a conventional problem, and therefore, has an object to provide a gm-C filter system having low power consumption.
To achieve the above-described object, according to a first aspect of the present invention, a transconductance-capacitance filter system comprises: a transconductance-capacitance filter circuit including a transconductance amplifier and a capacitor; an adjusting circuit including an oscillator containing a transconductance amplifier having the same structure as that of the transconductance amplifier of the transconductance-capacitance filter circuit, the adjusting circuit producing a digital adjusting value used to adjust the transconductance of the transconductance amplifier of the oscillator based upon an oscillation signal outputted from the oscillator; a register for holding the digital adjusting value supplied from the adjusting circuit; and a D/A converter for converting the digital adjusting value held in the register into an analog adjusting value which is used to adjust the transconductance of the transconductance amplifier of the transconductance capacitance filter circuit; wherein the adjusting circuit is operated in an intermittent manner.
A transconductance-capacitance filter system, according to a second aspect of the present invention, transconductance further comprises a temperature sensing circuit for sensing an ambient temperature of the transconductance-capacitance filter system, and wherein the adjusting circuit is operated in the intermittent manner based upon a change contained in the ambient temperatures.
A transconductance-capacitance filter system, according to a third aspect of the present invention, transconductance further comprises a power supply voltage sensing circuit for sensing a power supply voltage of the transconductance-capacitance filter system, and wherein the adjusting circuit is operated in the intermittent manner based upon a change contained in the power supply voltages.
A transconductance-capacitance filter system, according to a fourth aspect of the present invention, transconductance further comprises a temperature sensing circuit for sensing an ambient temperature of the transconductance capacitance filter system, and a power supply voltage sensing circuit for sensing a power supply voltage of the transconductance-capacitance filter system, and wherein the adjusting circuit is operated in the intermittent manner based upon either a change contained in the ambient temperatures or a variation of the power supply voltages.
Also, according to a fifth aspect of the present invention, a transconductance-capacitance filter system comprises: a transconductance-capacitance filter circuit including a transconductance amplifier and a capacitor; an adjusting circuit including an oscillator containing a transconductance amplifier having the same structure as that of the transconductance amplifier of the transconductance-capacitance filter circuit, the adjusting circuit producing a digital adjusting the transconductance of the transconductance amplifier of the oscillator based upon an oscillation signal outputted from the oscillator; a register for holding the digital adjusting value supplied from the adjusting circuit; a D/A converter for converting the digital adjusting value held in the register into an analog adjusting value which is used to adjust the transconductance of the transconductance amplifier of the transconductance-capacitance filter circuit; and a temperature compensating circuit for producing such a drive bias current capable of compensating for a variation component of the transconductance values of the transconductance amplifier of the transconductance-capacitance filter circuit with respect to a change contained in ambient temperatures of the transconductance-capacitance filter system based upon externally-supplied temperature data, and capable of driving the D/A converter by the drive bias current, wherein the adjusting circuit is operated only when the transconductance capacitance filter system is initiated.
Further, according to a sixth aspect of the present invention, a transconductance-capacitance filter system comprises a transconductance-capacitance filter circuit including a transconductance amplifier and a capacitor; an adjusting circuit including an oscillator containing a transconductance amplifier having the same structure as that of the transconductance amplifier of the transconductance-capacitance filter circuit, the adjusting circuit for producing a digital adjusting the transconductance of the transconductance amplifier of the oscillator based upon an oscillation signal outputted from the oscillator; a register for holding the digital adjusting value supplied from the adjusting circuit; a temperature compensating circuit for producing such a drive bias current capable of compensating for a variation component of the transconductance values of the transconductance amplifier of the transconductance-capacitance filter circuit with respect to a change contained in ambient temperatures of the transconductance-capacitance filter system based upon externally-supplied temperature data, and capable of driving the D/A converter by the drive bias current; an adder for executing a digital calculation with respect to the compensating digital adjusting value supplied from the temperature compensating circuit and the digital adjusting value held in the register; and a D/A converter for converting a digital calculation result supplied form the adder into an analog adjusting value which is used to adjust the transconductance of the transconductance amplifier of the transconductance-capacitance filter circuit, wherein the adjusting circuit is operated only when the transconductance-capacitance filter system is initiated.